This invention relates to bar code scanning, and in particular to techniques for combining, or stitching, partial scans.
A bar code symbol is a coded pattern of indicia comprising a series of bars and spaces having different light reflecting characteristics. Bar code scanning systems electro-optically transform the indicia into electrical signals, which are decoded into alphanumerical characters. Characters are typically represented in digital form and are provided as an input to a data processing system for applications, such as point-of-sale processing and inventory control. Scanning systems of this general type have been disclosed, for example, in U.S. Pat. Nos. 4,251,798; 4,369,361; 4,387,297; 4,409,470; 4,760,248; and 4,896,026, all of which have been assigned to the same assignee as the instant application.
Bar code symbols are formed from a series of bars and spaces, called elements, which are typically rectangular in shape and have a variety of possible widths. The specific arrangement of elements defines the character represented according to a set of rules and definitions. To encode a desired sequence of characters, groups of elements are concatenated to form a bar code symbol, with each character of the message represented by a corresponding group of elements. In some symbologies a "start" and "stop" character is used to indicate where the bar code begins and ends. There are a number of symbologies in use, e.g., UPC/EAN, Code 39, Code 128, Codabar, and Interleaved 2 of 5.
In typical scanning systems, a light source, such as a laser or laser diode, produces a beam which is directed by a lens or similar optical components along a light path toward a target that includes a bar code symbol on the surface. The beam produces a spot on the target. To scan with a laser system, the spot may be moved with an oscillating motor to produce a line or series of lines across the symbol. A portion of the light that is reflected off the symbol is detected by a sensor which may be positioned in the scanner. The sensor converts the reflected light into an electrical signal which is converted to a digital representation by electronic circuitry. For example, an analog electrical signal from the photodetector may be converted into a pulse width modulated digital signal, with pulse widths corresponding to the physical widths of the bars and spaces.
The decoder receives the pulse width modulated digital signal from the scanner, and attempts to decode the scan. If the start and stop characters and all of the characters between them are decoded successfully, the decoding process is finished. Otherwise, the decoder receives a next scan and attempts to decode it. Each attempt is based on the signal received during a single scan. The process continues until a scan is completely decoded or until no more scans are available.
A scan line or series of scan lines may not always entirely cross the bar code symbol. For example, a scanner may be in a fixed position next to a conveyor with containers, each of which has a bar code symbol printed on a label. If the label is skewed with respect to the scan line, there may be no single scan line which includes both the start and stop characters. These incomplete scan lines, called fragments, were discarded until techniques were developed to stitch or combine the fragments together, so that the decoding process can be completed.
Several basic techniques have been employed to combined fragments. One technique, known as block decoding, involves combining predefined regions or blocks in a number of scans. For example, a symbology such as UPC has start, stop, and center characters. If one fragment includes a start and a center, and another fragment includes a center and stop, these two "blocks" can be combined to form a complete scan.
In another method, a system checks fragments for corresponding portions, and stitches by superimposing. For example, the widths of elements can be measured in clock pulses for two fragments. These widths may be converted to a binary representation of widths or of ratios of widths between successive elements. The pulse count or other representation of the width of an element in one scan is compared to the width of an element in another scan. If these are the same or sufficiently similar, adjacent elements are compared. If a group of elements of a certain length are the same, the two groups of elements are considered to be corresponding, and the two fragments are stitched by superimposing the common portions.